Hi, I want to move a Gameobject to fit my ARCamera. I attach the GO to the camera, but it depends on its size, the GO should be further or closer on the Z axis. How can I find this relationship without scaling the GO?

You will need to find the closest distance Z at which the bounding box of the game object does not intersect against the camera frustum. A simple brute force solution would be to subdivide the space and check to see if the bounding volume intersects the rays representing the frustum edges until they no longer do. A more elegant solution would be determine the rate of change in the farclip plane’s area and apply it as a function of Z then compute the limit as the area approaches the area of the largest bounding box face on the game object.